It is definitely true that the frequency response
of a partial line source depends on distance.
(I mentioned this earlier but perhaps without
This is part of the reason that some theoretically
inclined people, eg Stanley Lipshitz, object to them.
Maybe I can explain(without the mathematics)
roughly why the distance matters.
A point source drops off like 1/distance squared
A (true) line source drops off like 1 /distance.
Now a finite length line source is partly like
a point source at low frequencies. (Imagine a really
low frequency --the finite line source will not
be at all large relative to the wavelength of the sound
so the line source is really acting like a point)
But as the wavelength gets shorter(as frequency rises)
then the line source really starts to act like a line
Now you can see why the balance shifts with distance:
lower frequencies tend to drop off more like 1/distance squared
being more point source like while high frequencies
being really like a line source, drop off like a/distance.
Hence at larger distance, more highs in proportion to lower down!
Hope that makes sense to everybody!
--- In firstname.lastname@example.org, Kim Rochat <krochat@...> wrote:
> Thanks for the comprehensive response. From your
> comments it seems like I shouldn't try to use the
> BG drivers near-field if moving from 8 to 10+ ft
> made a significant improvement for you.
> I was going to mount them in a wooden frames,
> height adjusted to center my ear on the driver.
> Crossover to the corner woofers would
> be around 325Hz, with crossover, delay, and equalization done using Acourate.
> I don't need much loudness - I'm trying to
> protect my aging ears, so that's not a factor.
> I will have to rethink placement. It seems that
> the 40" driver would need at least 8 feet in front + probably 7 feet behind
> Thanks again,
> At 05:24 PM 2/25/2013, you wrote:
> >Hi Kim,
> >Look a little further down, in that same table,
> >at the power handling specification. It appears
> >that the only difference in specifications as
> >you go into the larger ribbons, is greater power
> >handling ability. which I suppose translates
> >into greater max SPL given the same crossover
> >point. I don't listen particularly loudly, so
> >that difference wouldn't concern me.
> >I did reposition the speakers and listening
> >chair this evening, so that I am a little
> >further than ten feet away, as apposed to the 8
> >feet I was previously. There are a number of rather noticeable effects,
> >- Initial attack on piano keys, cymbals, drums and steel drums is a
> >lot more prevalent.
> >- The ephemeral quality that I had earlier
> >reported on some instruments is completely gone.
> >- The higher-than--the-head image height issue that I reported on some
> >instruments disappears and the entire listening height comes back down
> >to normal levels.
> >- More high frequencies energy.
> >- Less low frequency energy.
> >- Better tonal balance across the board.
> >I am thinking the the high / low frequency
> >energy balance comes into play because of the
> >traditional driver ported woofer that the planar
> >is mated to. Given that the planar driver loses
> >3 dB of SPL as compared to the 6 dB of the cone
> >as distance doubles, there must have been some
> >specific listening range that brings the two
> >into balance. Most of the things I pointed out
> >shouldn't matter if you are going to DSP the system.
> >The reason I even started looking into this
> >issue was that as I was listening last night, I
> >noticed rather small movements of my head were
> >having rather large results in treble energy,
> >and I thought that was not supposed to be an
> >issue with a narrow ribbon that has good
> >dispersion. When I moved the listening position
> >back, that issue also completely disappeared. I
> >think that ephemeral quality I was referring to
> >might have something to do with the relation of
> >the height line source and seating
> >distance. Since you mentioned the Tacts are
> >"corner woofers", I imagine you are not mounting
> >the panel on top of the woofer -as is the case
> >of the Soundlines. Maybe you have the ability
> >to mount the ribbon at a lower height, which
> >might avoid some of the issues with treble
> >energy stability, image height, and
> >"ephemeralness". All of the other frequency
> >balance issues should go away with DSP.
> >Of course, I don't know the science of all of
> >this, and the reported differences might be due
> >to something completely different. I can say
> >that the sound of the ribbons don't need any EQ
> >once I've moved the listening position back. It
> >might be due to distance needed for the ribbon
> >to integrate with itself, or to integrate with the bass woofer.
> >I would be very interested in your results if
> >you do buy the ribbon and try a home grown
> >speaker. Also note that the manufacturer
> >recommends a notch filter between 5 and 6 kHz to
> >deal with the resonance of the panel. I would
> >like to know how wide and deep that filter
> >should be, as I was considering an experiment
> >bypassing the passive crossover and using the
> >Holm unit's digital crossover and EQ capabilities instead.
> >Maybe Robert can answer your question regarding
> >an inherent characteristic of the quasi-ribbon that inhibits treble EQ.
> >On Mon, Feb 25, 2013 at 7:18 PM, Kim Rochat
> ><<mailto:krochat@...>krochat@...> wrote:
> >At 03:37 PM 2/25/2013, you wrote:
> > >I learned something interesting today regarding the diving high
> > >frequency response I reported in my measurement of the Soundline
> > >speakers. The manufacturer of the panel used in the speaker
> > >recommends a minimum listening distance of 10 feet for this specific
> > >model, in order to avoid high frequency losses. My listening room
> > >will support such distances, but I never thought to try such a
> > >distance. I'll try repositioning tonight.
> > >
> > >Alan
> >The driver in your Soundline is a Bohlender Graebener RD50.
> >I've been looking at the BG spec sheet for the various models of
> >drivers and it says:
> >"The RD drivers are essentially line-source
> >radiators, hence there is a certain
> >relation between the driver length and listening distance, as
> >outlined the table
> >below. Smaller listening distances may result in a somewhat subjective
> >perception of decreased output at the highest frequencies. This
> >limitation can be
> >overcome in smaller systems with a complementing supertweeter."
> >"Minimum recommended listening distance
> >Model RD75 (75" tall - 15 ft.)
> >Model RD50 (50" tall - 10 ft - this is the model used in the Soundline)
> >Model RD40 (40" tall - 8 ft)"
> >I've been listening to my Vandersteens near-field - about 4' and
> >they're well away from the side and back walls.
> >I've been toying with the idea of replacing the Vandersteens with a
> >pair of BG drivers and using them with my TacT corner woofers.
> >But this listening distance thing has me confused. All three models
> >list the same frequency response. I'm thinking that the 40" model
> >(which is also the cheapest) would be the one to get. Assuming it's
> >mounted vertically (not tilted) and I'm listening on-axis, both
> >horizontally and
> >vertically - is there an advantage to using the taller drivers? Also,
> >can the treble can be equalized to whatever I want if I'm listening
> >closer than the recommended distance or is there some characteristic
> >of the ribbon that inhibits treble equalization?